Scanners are used to scan an image to create a scanned image which can be displayed on a computer monitor, which can be used by a computer program, which can be printed, which can be faxed, etc. One conventional method for scanning an image uses a scanner having a subscan axis, a scan bar having sensor elements (such as CCD [charge-coupled-device] elements), and a scan-bar shading calibration strip having a white area and a black area.
It is noted that each optical sensing element produces a signal proportional to the amount of light reaching the element. The proportion or “gain” of each element is related but not identical. In addition, the light source may not uniformly illuminate the document to be scanned. To get an image with a consistent representation, the elements must be individually calibrated (also referred to as “shaded”) using a calibration strip with a white area and a black area.
To perform shading, the scan bar, including the sensor elements, is moved along the subscan axis over the white area and over the black area of the shading calibration strip, and white and black reference values of the output signal of the sensor elements are obtained. A gain is calculated for a particular sensor element by dividing the difference between desired white and black reference values by the difference between the average white and black reference values for the particular sensor element. Known techniques for calculating an average value for the white reference values of one CCD element include finding the median value, eliminating white (and/or black) reference values from optical defects on the white (and/or black) area of the shading calibration strip when doing the calculation, and calculating an average black value of the one CCD element. The one CCD element is shaded by calibrating a non-reference value of the output signal of the one sensing element by subtracting an offset, equal to the black average value of the one CCD element, from the non-reference value and then multiplying by the gain. Shading of the CCD elements compensates for varying amounts of illumination produced by a scanner light source in different regions of the scanned image and compensates for variations among the CCD elements of the scan bar. However, typically the black reference values are much noisier than the white reference values resulting in a too high offset which will produce unwanted darker vertical streaks when a scanned image is displayed and/or printed.
Another conventional method uses a shading calibration strip without a black area and obtains black reference values by not illuminating the shading calibration strip.
What is needed is an improved method for shading an optical sensing element such as an optical sensing element of a scanner.